METHOD OF COUNTING BACTERIA 



73 



a simple calculation the total number of colonies present can be 

 obtained. Plate-cultures in Petri's dishes are sometimes em- 

 ployed for purposes of counting. The bottoms of such dishes 

 are, however, never flat, and the thickness of the medium thus 

 varies in different parts. If these dishes are to be used, a circle 

 of the same size as the dish can be drawn with Chinese white on 

 a black card, the circumference divided into equal arcs, and radii 

 drawn. The dish is then laid on the card, the number of 

 colonies in a few of the sectors counted, and an average struck 

 as before. In counting colonies it is always best to aid the 

 eye with a small hand-lens. 



Method of Counting Living Bacteria in a Culture. This 

 is accomplished by putting into practice a dilution method 



such as that described on p. 58. 

 Measured amounts of high dilations 

 are plated, and the numbers of 

 colonies which subsequently develop 

 are counted. In applying such a 

 method it is necessary to have pipettes 

 capable of measuring small quantities of 

 fluid. Those discharging '05 and '1 c.c. will 

 be found convenient, and such pipettes can 

 have subdivisions which enable them to be 

 used for measuring still smaller fractions of 

 a cubic centimetre. Pipettes of this kind 

 can be obtained at the instrument makers. 

 Wright has described a method by which 

 a pipette (Fig. 29) for measuring small 

 quantities of fluid can be made from ordinary 

 quill tubing. The method is as follows : 

 A piece of quill tubing about 15 cm. long is 

 drawn out to a capillary stem. A standard 

 5 c.mm. pipette (such as that of the Gower's 

 h<emocytometer), or the pipette described 

 later on p. 119, is filled with mercury and 

 the metal transferred to the capillary stem 

 and run down to near its extremity; the 



, upper and lower limits of the mercury are 

 tic. 29. Wrights i j .-, ! -i .LI 



250 c.rnm. pipette marked with an oil pencil ; the mercury is 



fitted with nipple, then displaced up the tube till its previously 



distal end is at the proximal of the two 



marks, and a third mark is made at the new position of 



the upper end of the droplet; the manipulation is repeated 



three more times, and finally the tip of the tube beyond the 



250 c. mm, 



2Z5 



25 



20 



15 



10 



2-5 



